Setting up PC to Dual Boot

I just found out I can run a different OS on my eMachine and was wondering how to set up my system to dual boot....i think that's what it's called when you select what OS you want to use if you have more than one. I have Windows XP home now and want to use Ubuntu( linux based ) as a second if that's possible. Any help would be greatly appreciated.

dual boot is a headache to maintain
just build another box to play with a different o/s on
trust me i'm a professional